1. Field of the Invention
The present invention relates to a method for producing a grain-oriented electrical steel sheet having an ultra low watt loss. More particularly, the present invention relates to a method for producing a grain-oriented electrical steel sheet having an ultra low watt loss, in which the improvement of the watt loss due to a subdivision of magnetic domains does not disappear even after stress relief annealing is carried out during, for example, the production of a wound core.
2. Description of the Related Art
The grain-oriented electrical steel sheet is used mainly as the core material of transformers and other electrical machinery and devices, and must, therefore, have excellent excitation and watt-loss characteristics. In the grain-oriented electrical steel sheet, secondary recrystallized grains are developed which have a (110) plane parallel to the rolled surface and a &lt;001&gt;axis parallel to the rolling direction. These grains have the so-called Goss texture formed by utilizing the secondary recrystallization phenomenon. Products having improved exciting and watt-loss characteristics can be produced by enhancing the orientation degree of the (110) &lt;001&gt;orientation and lessening the deviation of the &lt;001&gt;axis from the rolling direction.
Note, the enhancement of the (110) &lt;001&gt;orientation leads to a coarsening of the crystal grains and an enlargement of the magnetic domains due to a passing of domain walls through the grain boundaries. There occurs, accordingly, a phenomenon such that the watt loss cannot be proportionally lessened to enhance the orientation.
Japanese Examined Patent Publication No. 58-5968 proposes to lessen the watt loss by eliminating the nonproportional phenomenon regarding the relationship between the orientation enhancement and the watt loss-reduction. According to this proposal, a ball or the like is pressed against the surface of a finishing-annealed, grain-oriented sheet so as to form an indentation having depth of 5 .mu. or less. By this indentation, a linear, minute strain is imparted to the steel sheet, with the result that the magnetic domains are subdivided.
Japanese Examined Patent Publication No. 58-26410 proposes to form at least one mark on each of the secondary recrystallized crystal grains by means of laser-irradiation, thereby subdividing the magnetic domains and lessening the watt loss.
The materials having ultra-low watt loss can be obtained, according to the methods disclosed in the above Japanese Examined Patent Publications Nos. 59-5868 and 58-26410, by imparting a local minute strain to the sheet surface of a grain-oriented electrical steel sheet.